This invention relates to an improvement in a hydraulic percussion drill for drilling rocks or the like by its shank rod or chisel percussed by a striker piston adapted to reciprocate by a hydraulic pressure.
The hydraulic percussion drill of the kind specified which has heretofore been employed comprises a striker piston and a pilot valve formed as an integral unit thereof, and the arrangement is made such that the reciprocation of the striker piston generates a pilot pressure and the change-over of a hydraulic change-over valve by the pilot pressure causes continuous reciprocation of the striker piston. The abovementioned conventional hydraulic percussion drill has a pilot port for the pilot valve formed in the upper part of a cylinder in which the striker piston is accommodated. Such conventional hydraulic percussion drill is disadvantageous in that if it is put to use for an extended period of time the striker faces of the striker piston and the shank rod tend to wear down so as to cause reciprocation of the striker piston beyond its optimum stroke thereby causing incorrect operational relationship between the pilot valve spool and the port so that a predetermined percussion energy cannot be obtained.